Field of the Invention
This invention pertains to the general field of optical metrology. In particular, it pertains to a novel method and apparatus for measuring the orientation of machined grooves in manufactured parts f or the purpose of quality control during the manufacturing process.
Description of the Prior Art
Many industrial parts are manufactured or finished by a process where a cutting tool removes material from the part, thereby shaping it and/or smoothing it. Milling, turning, grinding, and boring are such machining processes where the motions of the cutting tool and the workpiece relative to each other, referred to in the art as “feed” and “cutting speed,” respectively, produce the finished part. The shape of the tool and its penetration into the surface of the workpiece, combined with these motions, yield the desired shape of the resulting work surface.
In the formation of finite surfaces, some form of turning and translating of a single cutting edge or broad contact area are used to remove material from a rotating workpiece. While the workpiece rotates, the cutting tool moves slowly in a predetermined direction and removes material from the surface of the rotating workpiece. In more complex cases, the translation in the predetermined direction can be associated with a secondary translation in a perpendicular direction in order to accommodate more advanced geometries. As the contact area is from one or more locations on the machining tool, the tool necessarily leaves one or more grooves on the workpiece. The groove or grooves lie in a plane substantially normal to the main axis of the part (around which the part is rotated during milling), but not exactly so because the advancing feed motion of the cutting tool and the rotational speed during milling necessarily produce a groove orientation with a particular angle with respect to the axis of rotation. In fact, the grooves substantially define a helix characterized, by definition, by the fact that the tangent line at any point makes a constant angle with the main axis of the part. In the context of machining grooves, this angle is normally referred to as the lead angle of the groove or lead mark. In many cases, the angle is desired to be as close to perpendicular to the rotational axis as possible, while in other cases a specific direction of the grooves is desired, such as to ensure material always flows in one direction as the part is actuated in its final application.
When a cylindrical part so produced is used in a lubricated rotating application, such as in a bearing, the presence of grooves that are not perfectly perpendicular to the axis of rotation produces a pumping action that transports the lubricant from one side of the part to the other, thereby either depleting the lubricant from its operating environment or introducing a foreign fluid from the exterior, depending, as one skilled in the art will readily understand, on the direction of rotation of the part and the orientation of the groove relative thereto. In either case, this is a problem that can be serious in applications where the retention of uncontaminated lubricant is critical, as in automotive applications. The presence of seals is typically not sufficient to overcome this problem.
Therefore, during the manufacturing of these parts, it has become important to measure key properties of these grooves, including lead angle, depth, orientation, and frequency to ensure that they are kept within acceptable tolerances for the particular application of interest. (Note that a minimal lead angle is unavoidable in a part finished with a lathe because of the feed motion of the cutting tool.) If the angle is not within a predetermined tolerance of a design value, for instance, the manufacturing process is stopped and the part is removed from the process and is either discarded or re-machined, as may be appropriate, after calibration or repair as of the cutting tool. Among the methods used to measure tolerance parameters, for example, the automotive industry has relied on a simple technique applicable only to cylindrical parts. It consists of placing a thin string or thread in the groove of the perfectly horizontal part, rotating the part, and measuring the axial shift of the thread after a known number of rotations. (See http://www.bsahome.org/tools/pdfs/Wear_Sleeves_web.pdf.) From this information and from the knowledge of the dimensions of the part, the angle of the groove with respect to the part's axis is easily calculated. For instance, if a part with diameter D shows an axial shift/of the thread placed in the groove for each turn of the part (i.e., the pitch of the helix defined by the groove), the angle of the groove with respect to the part's axis will be easily calculated as arcsin(2l/D). (While this relation is not exact, one skilled in the art will appreciate that it is nonetheless a very close approximation for small angles.)
However, this simple measurement technique can only work for cylindrical parts when the groove is pronounced enough to hold and translate the measurement thread, which is not always the case and is rarely so for parts intended to be perfectly smooth, such as the surface of a bearing. In addition, the technique requires that the part be rotated around an axis substantially coincident with its main axis, which is time consuming and difficult to achieve in a test setting; it is slow to carry out because of the thread and part manipulations involved; and it is not suited for the automated quality-control needs of modern industrial manufacturing applications. Lastly, the measurement of motion of the string is inexact and highly susceptible to operator error, making the measurement non-repeatable and of insufficient accuracy for many modern applications. The present invention is directed at providing an optical approach that overcomes these drawbacks.